You don't say what focal length was used, ie wide or long, but f4.5 should provide adequate DoF for the scene. To reduce the EV value, it would be better to increase the SS than to make the aperture smaller, certainly don't go smaller than f8.
However, scenes like this are extremely difficult to control with a digital camera, it's all about the DR and the straight line response of the sensor system.

I was a bit worried when the competition started as I looked out at the sky and could not tell the difference between it and a neutral gray test card. Luckily the skies cleared in time for people to take some shots and make their entries.